Material pellets are used for research in a wide range of technologies. For example, in semiconductor and thermoelectrics technologies, knowledge of the transport characteristics of a material, including its electrical resistivity (DC and AC), Hall coefficient, thermal conductivity and thermopower is required. In addition, in the area of structural materials, accurate information regarding a sample's toughness, yield strength, and hardness is often required. In the powder metallurgy field, reliable information on material properties can be obtained from a highly dense pellet (e.g., a pellet having greater than 99% density and less than 1% air pockets). Furthermore, pellets of materials are often used to obtain greater understanding of a material through optical measurements such as ultraviolet-visible (UV-Vis), infrared (IR) or magnetization measurements. Pellets are used in a wide variety of applications. In any given application, there may be many different material types to be researched, each of which has a myriad of chemical and processing variations.
Powdered material is often used as a starting form for making components of complex shapes. Powdered material is transformed into a dense, solid body through the application of pressure and/or heat. The general method for creating a dense body begins with loading loose powder into a die. The powder can be a metal, a ceramic, a plastic or any other material that is to be compressed. Pressure is applied to the powder through loading of an upper and lower punch. This pressure is high enough to cause the powdered material to fuse and take the shape of the interior of the die. If the load is taken off, the part can be removed from the die as a solid body. In this green state, the powder is usually not fully dense, the part lacks cohesion and is either very brittle or remains powdery. A green body is converted into a dense body by consolidation, a process that removes voids from the pellet, thus increasing the density. Consolidation requires mass transport within the green body, a process that can be activated by heat (sintering), ultra-high pressure, and/or the application of a voltage between the punches (e.g., Spark Plasma Sintering).
Spark Plasma Sintering (SPS) achieves consolidation through the application of a potential difference between the upper and lower punches. Advantages to this process include the reduction of sintering time and, as a consequence, the ability to retain the nanostructured grain structure necessary in many applications. The process of SPS is achieved by application of a potential difference (˜5 Volts, for example) between the punches and the generation of very high currents (>1000 Amps, for example). These currents are thought to induce consolidation by generation of heat via Joule heating and through the generation of plasma within the powder material.
Uniaxial hot pressing is used to densify a loose powder into a solid body, such as a pellet, that will be tested to determine different properties of the densified material. A load is applied to a powder (held in a die) in one direction, thus compacting the powder. At the same time, heat is applied to the powder in order to sinter it and bring the bulk component close to 100% material density. Traditional uniaxial hot pressing methods process one material sample at a time. In a lab where a large number of material samples are required for testing, this is too slow and expensive.